John Stumbles wrote:
On Sun, 10 Aug 2008 14:54:47 +0000, Stephen wrote:

Hello,

When fitting a boiler, how do I work out what output the boiler should
have? I can add up the rating of the radiators to give the power
needed for the central heating but what do I add for the hot water?
Also do I add a fraction to compensate for losses?

Current practice for sizing boilers is the whole-house calculation method
which you'll find on the diy wiki under central heating design.

quick'n'dirty is between 50W/sq meter (well insulated house to modern
standards) through 100W/sq meter (DG, cavity walls, but no cavity
insulation, insulated roof, decent carpets on slab floors) through
200W/sq meter (single brick, but in good order otherwise) to 400W (old
house, no insulation at all)

boiler an rad sizing is not critical as long ass there is ENOUGH the
thermostat will modulate down to what is required.

when in doubt go bigger.